The present disclosure generally relates to plasma ashing apparatus, and more particularly to tuning hardware for plasma ashing apparatus and methods of using the hardware.
Radio frequency or microwave (“microwave”) plasma generation equipment is widely used in semiconductor and industrial plasma processing. Plasma processing supports a wide variety of applications, including etching of materials from a substrate or workpiece, deposition of materials onto a substrate, cleaning a workpiece surface, and modification of a substrate surface. In a plasma discharge device, a gas is flowed through a plasma tube located in a microwave cavity, and a plasma is excited in the gas by microwave energy. This plasma, or the afterglow therefrom, is typically directed to a process chamber where the substrate or workpiece resides and is used to remove or deposit material from or onto the substrate.
One mechanism to generate a microwave plasma includes a waveguide having a magnetron launcher on one end, and an applicator at the other end with a plasma gas tube running through the waveguide. A microwave field is generated in this section of the waveguide, such that the electrical energy couples to the gas in the applicator to produce a plasma therein. This plasma comprises among other charged species, excited gas atoms and molecules creating a high energy reactive state. The amount of microwave power coupled into the plasma load can vary significantly, and is typically a function of the plasma conditions such as chamber pressure, gas composition and gas flow, as well as the mechanism of impinging electric fields on the plasma load. These conditions, and therefore microwave power absorption by the plasma as well as reflected microwave power, can vary as a function of time while a workpiece is being processed by the plasma. Hence, the plasma can be a highly variable load for the microwave energy coupled to the plasma. Precautions must be taken to counteract the variability of microwave energy absorption by the plasma should any of the above mentioned conditions change. Otherwise, the microwave-excited plasma is likely to be quite inconsistent and variable with regard to a number of parameters (particularly species flux density) as a function of time and space that can have a deleterious effect on the substrate. Maximizing the power transfer from the supply to the plasma load is known as tuning the microwave circuit, and may be accomplished by changing the size and position of tuning stubs or the location of a sliding short, and other similar mechanisms.
A disadvantage associated with many plasma discharge devices designed for material removal, such as removal of photoresist—also known as ashing, is that they are designed for use with only a single type of gas, e.g., oxygen, fluorine-containing gas, or a small set of gas mixtures. Current plasma source ashing systems typically operate with a so-called “fixed-tune” system or network. The system can be adjusted prior to initialization or during startup of the plasma ash tool in order to optimize plasma conditions, but once startup is completed, the plasma source operates within the prescribed process window (e.g., for the desired gas compositions, flow rate, pressure, and the like) without any requirement of additional tuning However, when a process using a different processing condition such as gas type, gas composition, chamber pressure, etc. is to be performed, the energy coupling hardware must be changed, and a new piece of equipment must be used, resulting in sometimes unacceptable costs for particular manufacturing processes.
Fixed-tune networks, therefore, fail to minimize reflected power once a prescribed operating window is breached. As mentioned previously, the microwave excited plasma may absorb significantly different amounts of microwave energy as a function of plasma conditions, such as gas composition, gas pressure, and the like. Specifically, if a new process gas, gas mixture, or gas pressure is required for optimal processing of the substrate, the reflected power may no longer be minimized by the fixed tune network. This can lead to significant stability control problems for the plasma generation equipment. In such a case, additional tuning of the tuning stub(s) and/or sliding short is required in order to reduce reflected power, which would allow maximum power transfer to the new plasma load brought on by the change in gas composition. On a typical fixed-tune system, this adjustment can only be done by utilizing a different plasma ash tool dedicated to the new gas chemistry and/or pressure. This is a particularly egregious problem when the change in gas composition or pressure is simply for one step in a multi-step process. Using a different plasma ash tool, or stopping the process to tune the existing plasma ash tool, can be time consuming, cost prohibitive, and in some cases, impossible.
The problems associated with fixed tune systems have been recognized in semiconductor and industrial plasma processing industry for quite some time, as demonstrated by commonly assigned U.S. Pat. No. 6,057,645, which discloses a plasma discharge device that may be used with different fill gases over a wide range of process conditions. This is accomplished by providing a device which is broadly tunable, so that an appropriate resonant microwave mode may be achieved even when different gases and different operating conditions are present. The invention of that patent provides dynamic tuning by defining at least one end of a longitudinally extending microwave cavity with a microwave trap, and arranging for the longitudinal position of the microwave trap to be adjustable. In accordance with a further aspect of that patent, the microwave power is coupled to the cavity with an antenna which extends into the cavity, the degree of insertion of which into the cavity is adjustable to provide a further tuning adjustment, so that coupling of the desired resonant microwave mode may be enhanced while the operating window is enlarged.
Thus, it can be seen that tuning hardware has been integrated into microwave plasma ashing apparatus in order to optimize microwave energy into the plasma load. However, this type of tuning, known in the art as “internal tuning” can be cost prohibitive. One simple embodiment of such tuning hardware that can be adjusted to optimize the coupling of microwave energy into the plasma load, and to enable a plasma discharge device to be used over a wide range of process conditions is a stub tuner, which can be repositioned or resized to limit the amount of power reflected back to the source from the plasma load. This type of tuning mechanism, known typically as “external tuning” is significantly simpler and less expensive than the internal tuners. To that end, one or more pieces of tuning hardware can form a tuning network configured to transform the impedance of the plasma load to an impedance substantially equal to the impedance of the microwave source with reference to an output port of the microwave source into the tuning network. Specifically, a tuning stub can be used to minimize reflected power from the plasma applicator, and an adjustable tuning stub can enable the use of varying gas chemistries, mixtures, pressures, and the like within a single plasma ashing apparatus and reduce costs for particular manufacturing processes.
Based on the foregoing, what is needed in the art is economical adjustable tuning hardware for a plasma ashing apparatus that enables use of varying gas chemistries, mixtures, pressures, and the like within a single apparatus. In particular, a plasma ashing system can be outfitted with an adjustable tuning stub for selectively reducing reflected power from the process chamber, and enable the use of varying gas chemistries, mixtures, pressures, and the like within a single plasma ashing apparatus and reduce costs for particular manufacturing processes.